Reddit mentions of Fretboard Theory: Complete Guitar Theory Including Scales, Chords, Progressions, Modes, Song Application and More.

Best $20 you'll ever spend.

And here's a one page "cheat sheet" I made for practice. Prints out nicely on standard 8.5 x 11.

The most basic explanation of chords is that they're made up of a root (1st), a 3rd and a 5th. Minor chords have a flatted (half step down) 3rd.

A good exercise is to harmonize a scale. That means you play the scale, not one note at a time, but by playing chords (constructed by using a note from the scale as the root). So, instead of simply playing the individual notes of the scale C,D,E,F,G,A,B, you play the chords C, Dm, Em, F, G, Am, Bdim. Note the 7 chord is diminished (flat 3rd and flat 5th).

That's also what is meant by "singing in harmony". The melody line is triple voiced, with a 3rd and a 5th, accompanying the root. To start, for purposes of exercise, your melody will be simply an ascending/descending scale.

Follow the major/minor pattern for chord progressions (see "cheat sheet", where lower case = minor, upper = major). Note that arpeggios are just the roots, 3rds and 5ths of the scale. So, if you know your arpeggios, creating chords from any root is super simple.

I recommend reading this book: http://www.amazon.com/Fretboard-Theory-Desi-Serna-ebook/dp/B004ASNACS

Get yourself a copy of Fretboard Theory and blast through it, not worrying too much about absorbing everything. On the first pass you just want to acquaint yourself with the concepts. Because everything is so interconnected it's helpful to have even a fuzzy snapshot of the big picture before really diving in. Then re-read it a second time, taking care to fully absorb everything.

Once you know your scales, how chords are constructed, what progressions work for what situations, etc. revisit some of those songs you already know how to play and deconstruct them. What key is the song in? What progression is being used? Which scales?

It's bad advice to tell you to practice X first, then Y second. You should be practicing everything consistently and focusing on improvement in areas where you're struggling. Only you will know what that is, once you get going. But as far as where to start, page one of whatever theory book you choose.

Many ITT are recommending private instruction, which is fine, but I'd suggest getting what you can out of a book that costs less than a half hour lesson before you start working with a teacher. Any teacher will only be taking you through the exact same material for the first six months, anyway, and for a lot more money.

I'm in the same position as you. I've pretty much only played acoustic rhythm because I just wanted to hear myself play songs that I know. 90% of my playing has just been looking up the chords to songs I know and playing the crap out of them, which is fun, but eventually you want to move on.

I got an electric not even a week ago and figured now was a good time to actually learn about what I'm doing as opposed to just playing it blindly. I bought this book last night and went through the first couple chapters this morning. All I can say is WOW. Whenever I tried to learn scales and shit my eyes glazed over 5 minutes into it, but something about the way this book presents it just clicks with me.

Like I said I'm still pretty much at the very beginning of the book but it seems like a winner. I think it has like 150+ reviews on amazon so check those out if you want.